The Swedish Patent Specification No. (Swedish Patent Application 8802230-6), having a corresponding U.S. Pat. No. 5,251,235, describes apparatus for the contactless transmission of information over a serial twin-cable data bus, comprising a clock line and a data line connected to a transmitter, a receiver and a demodulator. The transmitter includes an oscillator and a modulator which is intended to modulate the signal generated by the oscillator in response to the clock line signals and data line signals respectively. The invention defined in said Patent Specification is characterized in that the modulator is constructed to carry out on the signal generated by the oscillator two significantly separated modulation steps which do not coincide in time, therewith to form two mutually independent signal channels, of which a first is intended for data signals and the other is intended for the clock signals. The demodulator is constructed to demodulate signals which are received by the receiver and which have been modulated in the aforesaid manner and also to recreate said two signals. The demodulator has a clock-line output and a data-line output.
The aforesaid Patent Specification defines a method of separating data and clock lines, by using pauses in transmission as information carriers, wherewith, e.g., switching or marking of levels in the clock line triggers a first modulation step, which constitutes a short transmission pause, whereas switching or marking of levels in the data line triggers a second modulation step which constitutes two rapid, sequential transmission pauses.